In a coated magnetic recording medium, i.e., a so-called coated type magnetic recording medium comprising a non-magnetic support having provided thereon a magnetic layer containing ferromagnetic fine particles and a binder, an abrasive agent has been added to the magnetic layer to improve its magnetic characteristics. Improvements of durability of a magnetic recording medium, improvements of squareness ratio, reduction of the coefficient of friction adjustments of the decrease of reproduced outputs as well as head wear have been attained by the addition of the abrasive agents, as disclosed in U.S. Pat. No. 3,630,910. Suitable abrasive agents are fine particles having a Mohs' hardness of 6 or more and having an average particle size of 3.0 .mu.m or less. Specific examples thereof include .alpha.-alumina, silicon carbide, chromium oxide, cerium oxide, corundum, artificial diamond, .alpha.-iron oxide, garnet, emery (main component: corundum and magnetite), silica, silicon nitride, boron nitride, molybdenum carbide, titanium carbide, titanium oxide, etc. These abrasive agents are disclosed in U.S. Pat. Nos. 3,687,725, 3,007,807 and 3,041,196, British Patent No. 1,145,349 and German Patent No. 853,211.
When .alpha.-alumina having an optional particle size (e.g., 0.005 .mu.m to 5.0 .mu.m) is used as an abrasive agent in a magnetic layer, the gloss of the magnetic layer is inferior to that of a magnetic layer containing the other abrasive agents, thereby decreasing CS/N ratio (i.e., color S/N or chroma S/N) and a resulting magnetic recording medium become poor for practical use. However, incorporation of alumina into a magnetic layer has the advantage that the magnetic layer hardly impairs the running system, e.g., magnetic head is not worn. Therefore, many attempts have been made to put .alpha.-alumina into practical use, without the above described disadvantage (e.g., decrease of the CS/N ratio).
As a result of extensive researches to improve the performance of .alpha.-alumina, it has now been found that the disadvantages are caused by the presence of fine particles thereof.